1. Technical Field
The present invention relates generally to circularly polarizing reflective material made from single layer Cholesteric Liquid Crystal (CLC) film material having xe2x80x9csuperxe2x80x9d broad-band reflection and transmission band characteristics approaching 2000 nm, and also to various novel methods for fabricating and using the same in diverse applications.
2. Background Art
In the modern world, there are numerous applications which require circularly polarizing material having broad-band reflection and transmission characteristics. Such applications range from polarizing filters used in optical systems, to highly reflective pigments used in the manufacture of CLC-based paints and inks.
A detailed review of the prior art literature reveals that European Patent Application 94200026.6 entitled xe2x80x9cCholesteric Polarizer and Manufacture Thereofxe2x80x9d, published Jul. 20, 1994 and assigned to Philips Electronics, N.V. of Eindhoven, Netherlands (the xe2x80x9cPhillips referencexe2x80x9d), is the most relevant prior art reference as it discloses several methods on how to make a single layer CLC film material having broad-band reflection and transmission characteristics. In order to achieve its broad-band reflection and transmission characteristics, which are limited to about 400 nm, the Phillips disclosure requires adding a UV dye into the CLC mixture in order that the pitch of the CLC material change xe2x80x9clinearlyxe2x80x9d from its maximum value at one film surface to its minimum value at the other film surfaces, wherein the difference between the maximum pitch and minimum pitch is greater than 100 nm.
According to the first fabrication technique disclosed in the Phillips reference, prior art CLC polarizing material is formed from two polymerizable chiral and nematogenic monomers, each of which has a different reactivity. During polymerization of the mixture by means of actinic radiation, a linear variation in actinic radiation intensity (i.e. a linear actinic radiation intensity gradient) is realized across the optically active layer of film by introducing an ultraviolet (UV) absorbing dye into the original mixture. This linear radiation intensity gradient causes the most reactive monomer to be preferentially incorporated into the least reactive monomer to occur at the locations of the highest radiation intensity. As a result, at least one concentration gradient of free monomers is formed during polymerization, causing the monomer to diffuse from locations with a low monomer concentration to location with a high concentration. The monomers of high reactivity diffuse to locations where the radiation intensity is highest. This diffusion process results in an increase in reactive monomers in areas of the formed polymer material where, during polymerization, the radiation intensity is highest. As a result, the composition of the material varies in a direction transverse to the surfaces of the film such that a xe2x80x9clinear variationxe2x80x9d in the pitch of the molecular helices results in the layer formed by the polymer. The liquid crystal material is distributed linearly across the thickness of the film. This variation in pitch provides the optically active layer with a bandwidth proportional to the variation in the pitch of the molecular helices. In thin CLC film structures, the maximum bandwidth variation achievable using this prior art fabrication technique is about 400 nm.
According to the second fabrication method disclosed in the Phillips reference, the spontaneous diffusion of monomers into a polymerizable CLC film is followed by UV polymerization. This fabrication method is carried out by depositing a film of reactive monomers on the surface of a polymerized film of CLC material. The diffusion of monomers into the CLC film layer causes a concentration gradient in the layer before diffusion is halted. As a result, the original CLC layer swells slightly causing an increase in pitch of the molecular helices. This provides a concentration gradient which, in turn, results in a xe2x80x9clinear variationxe2x80x9d in pitch across the film thickness. Polymerization of the layer by actinic radiation halts diffusion providing abroad-band polarizer having reflection characteristics approaching 400 nm in thin CLC film structures.
Notably, in fabrication techniques disclosed in the Phillips reference described above, the two principal materials utilized in the starting mixtures thereof are characterized as monomers having different reactivities. Moreover, when a dye is not utilized in the fabrication processes of the Phillips reference, a diffusion gradient is not established and both of the principal materials are polymerized, resulting in a narrow band polarizer.
While the above described Phillips reference discloses several methods for fabricating CLC-based circularly polarizing film having reflection characteristics approaching 400 nm in thin film structures, such bandwidth characteristics are inadequate in numerous applications where bandwidth characteristics up to five times greater are required. Also, such prior art fabrication methods require that the constituent materials both be polymerizable, restricting the many types of commercially available material that can be used during manufacture.
Thus, there is a great need in the art for circularly polarizing film material having reflection and transmission characteristics over a bandwidth that is substantially greater than the bandwidth provided by all prior art circularly polarizing material.
Accordingly, it is a primary object of the present invention to provide a circularly polarizing material which has a reflection and transmission bandwidth characteristics that are substantially greater than the reflection and transmission bandwidth characteristics of prior art circularly polarizing CLC material.
Another object is to provide such circularly polarizing material, having reflection and transmission bandwidth characteristics approaching 2000 nm.
Another object is to provide such circularly polarizing material having improved spectral and band-pass position characteristics.
Another object is to provide circularly polarizing material having such bandwidth characteristics, and being realized in a single thin film of CLC material in which the pitch of the helices of the CLC molecules varies in a non-linear manner along the depth dimension (i.e. transverse to the surface) of the CLC film structure.
Another object of the present invention is to provide such circularly polarizing material in the form of an extremely broad-band polarizing ink and/or paint.
Another object of the present invention is to provide a palette of CLC-based colored paints and/or inks, for use in various color applications including painting, printing and the like.
Another object is to provide super broad-band circular (or linear) polarizers realized on CLC film structures and having notch-characteristics suitable for color filtering and imparting applications.
Another object is to provide super broad-band CLC polarizing films which have reflection and transmission bandwidths that are over twice the extent of the prior art CLC polarizing films of the same overall thickness.
Another object is to provide single layer polarizers of extremely broad-bandwidth wherein the liquid crystal component of the polarizer assumes a non-linear distribution across the thickness of the polarizer.
Another object is to provide such circularly polarizing material, wherein the liquid crystal material may be non-polymerizable or of low molecular weight in the present invention.
Another object is to provide a method of fabricating circularly polarizing material having extremely broad-band spectral reflection and transmission characteristics, low optical loss properties, high polarizing efficiency and low manufacturing cost.
Another object is to provide a method of fabricating circularly polarizing material having extremely broad-band spectral band-pass characteristics, low optical loss properties, high polarizing efficiency, simplified fabrication, and low manufacturing cost.
Another object is to provide a method of fabricating extremely broad-band polarizers using a mixture of polymerizable CLC, liquid crystal material and a photoinitiator, wherein during polymerization of the polymerizable CLC, the segregation rate of the liquid crystal material is constrained to be greater than the polymerization rate of the polymerizable CLC being polymerized.
Another object is to provide a method of fabricating extremely broad-band polarizers, in a way which does not employ ultraviolet dye during the manufacture thereof.
Another object is to provide a method of fabricating super broad-band circularly polarizing material, wherein the polymerizable CLC being polymerized is exposed to a non-linear (e.g. exponential) intensity gradient of actinic (e.g. UV) radiation by virtue of light loss within the polymerizable CLC medium, thereby causing a non-linear variation in pitch of helices of the CLC molecules therein.
Another object is to provide a method of fabricating super broad-band circularly polarizing material, using commercially available constituent cholesteric liquid crystal polymers and liquid crystal material.
Yet another object is to provide a method of fabricating free standing circularly polarizing film having reflection and transmission characteristics over a super broad-band extending up to about 2000 nm.
These and other Objects of the Present Invention will become apparent hereinafter and in the Claims to Invention.
In accordance with a first aspect of the present invention, circularly polarizing material of novel construction is disclosed. Unlike any prior art reflective circular polarizer, the circularly polarizing material of the present invention has reflection and transmission bandwidth characteristics that extend over a super broad-band of wavelengths (e.g. up to 2000 nm). The circularly polarizing material of the present invention is made from a film of polymerizable material having a cholesteric order (e.g. polymerizable CLC film), in which non-cross linkable liquid crystal molecules (e.g. having a nematic phase) are distributed in a non-linear fashion in a plurality of liquid crystal-rich and liquid crystal-depleted sites across the thickness of the polymerizable CLC film. Depending on the final spiral structure of the polymerizable CLC materials utilized, the resulting circularly polarizing films of the present invention will reflect either left-handed or right-handed circularly polarized light having wavelengths within the above-described super broad-band portion of the electromagnetic spectrum.
The super broad-band, circularly polarizing reflective material of the present invention can be used to fabricate numerous types of products, namely: super broad-band circularly polarizing panels; super broad-band color filters; super broad-band circularly polarizing pigment flakes used as colorants in inks and/or paints; glare-reducing sunglasses; micropolarization panels and polarizing eyewear used in SMI-based stereoscopic 3-D display and viewing applications; and the like.
According to a second aspect of the present invention, a novel method is disclosed for fabricating the super broad-band circularly polarizing material of the present invention. According to the method, a cholesteric liquid crystal (CLC) polymer (having a cholesteric order) is mixed with non-cross linkable liquid crystal material (e.g. having a nematic order), a photoinitiator, and a chiral additive which may or may not be chemically attached to the polymer at a temperature which maintains the mixture in a liquid state having a cholesteric order. Preferably, the liquid crystal material and polymerizable CLC chemically attached with the chiral additive (chiral group) are present in a ratio by weight of 1:2 but may be present in a ratio range by weight of 3:1 to 1:6 depending on all the conditions involved. Preferably, the photoinitiator is present in an amount of 0.6% by weight in the mixture but may be present in higher or lower amounts so long as the amount is sufficient to initiate polymerization of the polymerizable CLC material. In general, the amount of the photoinitiator introduced into the mixture should be such that, when the mixture is exposed to actinic radiation, the polymerization rate of the polymerizable CLC is slower than the segregation rate of the liquid crystal material.
While heating the mixture to, for example 92xc2x0 C., in order to retain its cholesteric order, the mixture is subjected to actinic radiation (e.g. ultraviolet light) for a time and at an intensity sufficient to polymerize the polymerizable CLC or the liquid crystal material or both. Because the actinic light has an exponential intensity distribution due to light attenuation by, for example, light absorption and/or scattering, polymerization occurs in a non-linear fashion, thereby resulting in a non-linear distribution of the polymer and the liquid crystal material across the film. During polymerization, phase separation takes place. The segregation rate of the liquid crystal material is designed to be greater than the polymerization rate of the polymerizable CLC being polymerized. Thus, the liquid crystal material segregates and diffuses to sites of enlarged pitch in the polymerizable CLC material from sites of shrunken pitch in the polymerizable CLC material. Consequently, an exponentially distributed pitch is generated from one surface to the other of the polymerizable CLC film. It has been demonstrated that the novel composition of this circularly polarizing reflective material provides for its markedly improved, super broad-band reflection and transmission characteristics.
While it is believed that any non-linear distribution of pitch of the helices of the liquid crystal material will produce super broad-band circularly polarizing material in accordance with the teachings of the present invention, the preferred embodiments thereof disclosed herein each have an exponentially distributed pitch extending from one surface of the polarizing film to the other surface thereof.